Understanding Celestial Mechanics & Timekeeping
The exacting mathematics beneath every predicted eclipse, spacecraft trajectory, and star chart — the laws of how bodies fall around one another, and the conventions that pin down where a thing is and when. Built on well-established mechanics and standards; only real constants are stated.
Related entities
- Theory
Kepler's Laws of Planetary Motion
Three laws describing how planets move: orbits are ellipses with the Sun at one focus; a line from the Sun to a planet sweeps out equal areas in equal times; and the square of a planet's orbital period is proportional to the cube of its…
- Orbital mechanics concept
Lagrange Points
The five points in a two-body system where a small third body can remain fixed relative to the two, its motion balanced by their combined gravity. The James Webb Space Telescope orbits the Sun–Earth L2 point, and the Trojan asteroids sit at Jupiter's L4 and L5.
- Reference frame
The International Celestial Reference System
The modern, fixed reference frame for the sky, defined by the positions of hundreds of distant quasars whose motion is undetectable. It replaced frames tied to the slowly-shifting equinox with one anchored to some of the most distant objects known — the standard to which all precise astronomical positions are now referred.
- Ephemeris system
JPL Development Ephemeris
The series of numerically-integrated ephemerides, produced by NASA's Jet Propulsion Laboratory, that give the positions and velocities of the planets, the Moon, and the Sun to high precision over long spans. The DE ephemerides are the standard reference for precise Solar System calculations.